Pipeline pigging

ABSTRACT

A system for transporting a pig (28) along the interior of a pipeline (26), particularly for the purpose of installing a liner (12) in the pipeline by towing the liner behind the pig, in which a hydrostatic head is applied on either side of the pig and the head on the downstream side of the pig is reduced such that the pig is transported along the interior of the pipeline by the resultant pressure differential. Where the system is employed in a pre-existing subsea pipeline, the hydrostatic heads may be applied by means of risers (14, 24) and the pressure differential created by means of a pump (30) adapted to pump water out of the downstream riser (24). The liner may be pulled through a flooded pipe by means of a pig, cable, self-propelled pipe crawler, or by combinations of these. A further cable may also be employed to apply a back tension, and/or to limit the pulling force applied to the liner and/or to enable the liner to be pulled backwards.

The present invention relates to improved methods and apparatus fortransporting a pig along the interior of a pipeline or the like,particularly but not exclusively a subsea pipeline. In its preferredform, the invention is particularly applicable to the use of a pig forthe purpose of installing a liner, typically of polymer material, in anexisting pipeline.

In accordance with a first aspect of the invention, there is provided amethod for installing a liner in a pipeline, comprising the steps offlooding the interior of the pipeline and pulling the liner along thelength of the interior of the pipeline from an upstream end thereof to adownstream end.

Preferably, said liner is pulled by means of a pig which forms asubstantially fluid-tight seal with the interior walls of the pipelineand in which a hydrostatic head is applied on either side of the pig,the method comprising reducing the hydrostatic head on a downstream sideof the pig such that the hydrostatic head on said downstream side isless than the hydrostatic head on an upstream side of the pig, wherebysaid liner is transported along at least part of said pipeline by theresultant pressure differential between said upstream and downstreamsides of said pig.

Preferably also, said pipeline is an existing subsea pipeline, an inletriser is connected to the upstream end of the pipeline in which theliner is to be installed, and an outlet riser is connected to thedownstream end of the pipeline, said risers each extending to the watersurface and said risers and the pipeline being open to hydrostaticpressure, the pig is introduced into the inlet riser, and said pressuredifferential is created by pumping water out of said outlet riser.

In accordance with a second aspect of the invention, there is providedapparatus for installing a liner in a pipeline, comprising meansflooding the interior of the pipeline and pulling means for pulling theliner along the length of the interior of the pipeline from an upstreamend thereof to a downstream end.

Preferably, said pulling means comprises a pig having fluid seal meansadapted to form a substantially fluid-tight seal with the interior wallsof the pipeline, means for applying a hydrostatic head on either side ofthe pig, and means for reducing the hydrostatic head on a downstreamside of the pig such that the hydrostatic head on said downstream sideis less than the hydrostatic head on an upstream side of the pig,whereby the liner may be pulled along at least part of said pipeline bythe resultant pressure differential between said upstream and downstreamsides of said pig.

Preferably also, said pipeline is an existing subsea pipeline, and saidmeans for applying a hydrostatic head on either side of the pig includesan inlet riser adapted to be connected to the upstream end of thepipeline in which the liner is to be installed, and an outlet riseradapted to be connected to the downstream end of the pipeline, saidrisers each extending to the water surface and said risers and thepipeline being open to hydrostatic pressure, in use, the pig beingadapted to be introduced into the inlet riser, and further includingpump means for creating said pressure differential by pumping water outof said outlet riser.

In accordance with a third aspect of the invention, there is provided amethod for transporting a pig along the interior of a pipeline, in whichthe pig forms a substantially fluid-tight seal with the interior wallsof the pipeline and in which a hydrostatic head is applied on eitherside of the pig, the method comprising reducing the hydrostatic head ona downstream side of the pig such that the hydrostatic head on saiddownstream side is less than the hydrostatic head on an upstream side ofthe pig, whereby said pig is transported along at least part of saidpipeline by the resultant pressure differential between said upstreamand downstream sides thereof.

In accordance with a fourth aspect of the invention, there is providedapparatus for transporting a pig along the interior of a pipeline,comprising a pig having fluid seal means adapted to form a substantiallyfluid-tight seal with the interior walls of the pipeline, means forapplying a hydrostatic head on either side of the pig, and means forreducing the hydrostatic head on a downstream side of the pig such thatthe hydrostatic head on said downstream side is less than thehydrostatic head on an upstream side of the pig, whereby said pig may betransported along at least part of said pipeline by the resultantpressure differential between said upstream and downstream sidesthereof.

These and other preferred and optional aspects and features of theinvention are defined in the Claims appended hereto.

Embodiments of the invention will now be described, with reference tothe accompanying drawings in which:

FIG. 1 is a schematic illustration of a liner being installed in asubsea pipeline, in accordance with a preferred embodiment of theinvention; and

FIG. 2 is a generalised, schematic illustration of pipe lining systemsin accordance with the invention.

FIG. 1 shows a length of pipeline 10 in which a liner 12 (typicallyformed from a polymer material such as HDPE) is to be installed. In thedrawing, the liner 12 is being fed from left to right; the left hand endof the pipeline 10 will be referred to as the "upstream" end and theright hand end as the "downstream" end. In this example, the liner isbeing installed in a pipeline which has previously been installed in asubsea location. It will be understood that the invention is alsoapplicable to pipelines other than subsea pipelines and/or to subseapipelines prior to installation of the pipeline, as shall be discussedfurther below.

In the illustrated example of FIG. 1, the liner 12 is fed into theupstream end of the pipeline 10 by means of a temporary, flexible, linerfeed pipe 14 (or "inlet riser") which extends from a support vessel 16on the surface 18 and is connected to the upstream end of the pipeline10 via an additional pipe section 20, which includes a valve 22 in itsside wall. In this example, the downstream end of the pipeline isconnected to an existing riser 24, via a further additional pipe section26.

The interior volume of the various pipes 10, 14, 20, 24 and 26 isflooded prior to insertion of the liner 12. The liner feed pipe 14 maybe perforated at its inlet below sea level for this purpose.

The valve 22, when open, allows communication between the interior andexterior of the pipe section 20. It may serve simply to open the pipe toexternal hydrostatic pressure or it may be connected to other equipment,either subsea or at the surface, such as dosing apparatus forintroducing additives to the pipeline, or to sources of fluids otherthan seawater. The valve is closed at the beginning of the liningoperation.

A pig 28 is connected by any suitable means to the leading end of theliner 12 and is fed into the upstream end of the liner feed pipe 14. Thepig 28 includes annular seals 29 adapted to form a substantiallyfluid-tight seal with the interior walls of the pipes through which itis to travel. Once the pig 28, towing the liner 12, has been insertedinto the liner feed pipe 14, a submersible pump 30, which has beenlowered into the existing riser 24, is operated to pump water out of theriser 24, thereby reducing the hydrostatic head on the downstream sideof the pig 28. The pressure differential thus created between theupstream and downstream sides of the pig 28 causes the pig to betransported along the length of the pipes 14, 20, 10, and 26. The liner12 itself is open at its ends and free-flooding.

Once the pig 28 passes the valve 22 in pipe section 20, the valve 22 maybe opened to hydrostatic pressure. This eliminates the possibility ofair being sucked into the inlet of the liner feed pipe 14. Thedownstream pipe section 26 includes a pig stop 32 which limits thetravel of the pig 28. Once the pig 28 reaches the stop 32, the head inthe existing riser is restored to equalise the pressure across the pig.Thereafter, the pipe sections 20 and 26 may be disconnected from thepipeline to allow the pig 28 to be retrieved and to allow the liner 12to be terminated in a suitable manner at the ends of the pipeline 10.Additional pipe sections (not shown) may be included between the pipesection 20 and the upstream end of the pipe 10 and between thedownstream end of the pipe 10 and the pipe section 26. Such additionalpipe sections may include components and features such as valves forventing the annulus between the lining and the interior of the pipe andarrangements for terminating the ends of the liner, as is well known inthe art, and would remain as part of the pipe 10 after removal of thepipe sections 20 and 26.

The same approach of reducing the pressure on the downstream side of thepig 28 may be employed for transporting pigs along lengths of pipelinefor purposes other than the installation of liners.

The example of FIG. 1 relates particularly to an to the installation ofa liner in an existing subsea pipeline, making use of an existing riserat the downstream end of the pipeline. The invention may also be appliedto existing subsea pipelines where there is no existing riser in place;e.g. by installing a temporary riser at the downstream end or or by anyother arrangement allowing the pressure on the downstream side of thepig to be reduced relative to the pressure on the upstream side. Asnoted above, the invention may also be applied to pipelines on land, inwhich case the pipe would be flooded during the liner installationprocedure, the required pressure differential being generated by anysuitable arrangement. Temporary pipe sections would be attached as inFIG. 1, configured to allow installation of the pig and liner. Thetechnique may be applied to existing land pipelines, or to pipelinesprior to installation thereof either on land or subsea.

FIG. 2 illustrates the system of the invention in more general termsthan the particular example of FIG. 1, wherein features in common withFIG. 1 are designated by like reference numerals. As previouslydiscussed, whether the liner 12 is being installed subsea or on land,the pipe is flooded and means, such as suitable pumps etc. (not shown)are provided to generate the required pressure differential across thepig 28. Variations are also possible, as follow:

(a) For the purpose of installing the liner 12, a cable tow might beemployed in place of the pig 28. In this case, there would be norequirement for the generation of a pressure differential.Alternatively, as shown in FIG. 2, a cable 34 may be connected to thedownstream end of the pig 28 to assist pressure-driven movement of thepig.

(b) The pig 28 might be replaced by a self-propelled "pipe crawler" unit40, shown schematically in FIG. 2, of a type which is generally known inthe art. The movement of such a unit might be assisted by a pressuredifferential or by a cable tow or both.

(c) As shown in FIG. 2, a cable 36 may be connected to the upstream endof the pig, allowing a constant back tension to be applied so as toallow better control of the installation process. This arrangement wouldalso enable the prevention of excessive stresses and strains beingapplied to the liner in the (unlikely) event that the liner becomesstuck, and would allow the liner and pig to be pulled backwards ifnecessary for any reason. An upstream cable of this type might also beemployed with cable tows, self-propelled crawlers or combinations asdiscussed at (a) and (b) above.

Improvements and modifications may be incorporated without departingfrom the scope of the invention.

What is claimed is:
 1. A method for installing a liner in a subseapipeline, the method comprising the steps of:flooding the interior ofthe pipeline so as to apply ambient subsea hydrostatic head throughoutthe length of the pipeline; pulling the liner along the length of theinterior of the pipeline from an upstream end thereof to a downstreamend; connecting said liner to a pig which forms a substantiallyfluid-tight seal with the interior walls of the pipeline, applying saidambient hydrostatic head on both sides of the pig, and reducing thehydrostatic head on a downstream side of the pig such that thehydrostatic head on said downstream side is less than the hydrostatichead on an upstream side of the pig, whereby said pig is transportedalong at least part of said pipeline by the resultant pressuredifferential between said upstream and downstream sides of said pig andsaid liner is pulled along the interior of the pipeline by said pig. 2.A method as claimed in claim 1, wherein the method further comprises thesteps of:arranging an inlet riser connected to the upstream end of thepipeline in which the liner is to be installed and an outlet riserconnected to the downstream end of the pipeline such that said riserseach extend to the water surface and said risers and the pipeline areopen to hydrostatic pressure, introducing the pig into the inlet riser,and creating said pressure differential by pumping water out of saidoutlet riser.
 3. A method as claimed in claim 2, wherein the step ofarranging the inlet riser connected to the upstream end of the pipelinecomprises the step of connecting a temporary, flexible pipe to saidupstream end of the pipeline.
 4. A method as claimed in claim 2, whereinthe step of arranging the outlet riser connected to the downstream endof the pipeline comprises the step of employing a pre-existing riserpreviously connected to the downstream end of the pipeline.
 5. A methodas claimed in claim 2, further including the step of connecting thedownstream end of the inlet riser to the upstream end of the pipelinevia a first temporary pipe section including a valve in a side wallthereof.
 6. A method as claimed in claim 5, further including the stepsof closing said valve when the pig is inserted into the riser inlet andopening said valve once the pig has passed downstream of the valve.
 7. Amethod as claimed in claim 2, further including the step of connectingthe downstream end of the pipeline to the upstream end of the outletriser via a second temporary pipe section including a stop for limitingthe travel of the pig.
 8. A method as claimed in claim 1, furtherincluding the step of supplementing the hydrostatic force for pullingthe liner by applying additional traction force to said pig by a cable.9. A method as claimed in claim 1, further including the step ofsupplementing the hydrostatic force for pulling the liner by applyingadditional traction force to said pig by a self-propelled pipe crawlerunit.
 10. A method as claimed in claim 1, further including the step ofproviding a cable extending in the upstream direction from thedownstream end of the liner, whereby a force may be applied in saidupstream direction.
 11. Apparatus for installing a liner in a subseapipeline, comprising:means for flooding the interior of the pipeline soas to apply ambient subsea hydrostatic head throughout the length of thepipeline; and pulling means for pulling the liner along the length ofthe interior of the pipeline from an upstream end thereof to adownstream end; wherein said pulling means comprises a pig having afluid seal means adapted to form a substantially fluid-tight seal withthe interior walls of the pipeline, means for applying said ambientsubsea hydrostatic head on both sides of the pig, and means for reducingthe hydrostatic head on a downstream side of the pig such that thehydrostatic head on said downstream side is less than the hydrostatichead on the upstream side of the pig, whereby the liner may be pulledalong at least part of said pipeline by the resultant pressuredifferential between said upstream and downstream sides of said pig. 12.Apparatus as claimed in claim 11, wherein said means for applying ahydrostatic head on both sides of the pig includes an inlet riseradapted to be connected to the upstream end of the pipeline in which theliner is to be installed, and an outlet riser adapted to be connected tothe downstream end of the pipeline, said risers each extending to thewater surface and said risers and the pipeline being open to hydrostaticpressure, in use, the pig being adapted to be introduced into the inletriser, and further including pump means for creating said pressuredifferential by pumping water out of said outlet riser.
 13. Apparatus asclaimed in claim 12, wherein said inlet riser comprises a temporary,flexible pipe.
 14. Apparatus as claimed in claim 12, wherein said outletriser comprises a pre-existing riser.
 15. Apparatus as claimed in claim12, further including a first temporary pipe section whereby thedownstream end of the inlet riser may be connected to the upstream endof the pipeline, said first temporary pipe section including valve meansin the side wall thereof.
 16. Apparatus as claimed in claim 15, whereinsaid valve means is closed when the pig is inserted into the riser inletand opened once the pig has passed downstream of the valve means. 17.Apparatus as claimed in claim 12, further including a second temporarypipe section, whereby the downstream end of the pipeline may beconnected to the upstream end of the outlet riser, said second temporarypipe section including stop means for limiting the travel of the pig.18. Apparatus as claimed in claim 11, wherein the hydrostatic force forpulling the liner is supplemented by an additional traction forceapplied by means of a cable.
 19. Apparatus as claimed in claim 11,wherein the hydrostatic force for pulling the liner is supplemented byan additional traction force applied by means of a self-propelled pipecrawler unit.
 20. Apparatus as claimed in claim 11, wherein a cable isprovided extending in the upstream direction from the downstream end ofthe liner, whereby a force may be applied in said upstream direction.21. A method for transporting a pig along the interior of subseapipeline, the method comprising the steps of:forming a substantiallyfluid tight seal between the pig and the interior walls of the pipeline,applying ambient subsea hydrostatic head on both sides of the pig, andreducing the hydrostatic head on a downstream side of the pig such thatthe hydrostatic head on said downstream side is less than thehydrostatic head on an upstream side of the pig, whereby said pig istransported along at least part of said pipeline by the resultantpressure differential between said upstream and downstream sidesthereof.
 22. Apparatus for transporting a pig along the interior of asubsea pipeline, comprising:a pig having fluid seal means adapted toform a substantially fluid-tight seal with the interior walls of thepipeline, means for applying ambient subsea hydrostatic head on bothsides of the pig, and means for reducing the hydrostatic head on adownstream side of the pig such that the hydrostatic head on saiddownstream side is less than the hydrostatic head on an upstream side ofthe pig, whereby said pig may be transported along at least part of saidpipeline by the resultant pressure differential between said upstreamand downstream sides thereof.